Process for para-ethyltoluene dehydrogenation

ABSTRACT

Para-ethyltoluene dehydrogenation catalyst compositions and processes for using such catalysts are provided. The catalyst compositions comprise a catalytically active iron compound, e.g., iron oxide; a potassium catalyst promoter, e.g., potassium carbonate; an optional chromium compound stabilizer, e.g., chromic oxide, and a magnesium compound, e.g., magnesium oxide. Utilization of particular amounts of magnesium compound in dehydrogenation catalyst compositions of this type will provide a catalyst especially suitable for promoting the selective dehydrogenation of para-ethyltoluene to form para-methylstyrene in improved yields.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of application Ser. No. 452,471, filedon Dec. 23, 1982, now U.S. Pat. No. 4,496,662, issued on Jan. 29, 1985.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improved catalysts for the selectivedehydrogenation of dialkyl aromatic hydrocarbons to produce alkyl vinylaromatic hydrocarbons, more particularly to catalysts for the productionof para-methylstyrene (PMS) via the dehydrogenation of para-ethyltoluene(PET).

2. The Prior Art

The vinyl benzenes play a particularly important role in the preparationof synthetic plastics and resins. The polymerization of styrenes, forexample, to produce polystyrene resins is well known.

Styrene and styrene derivatives are typically produced from ethylbenzenematerials by dehydrogenation over solid catalysts in the presence ofco-fed steam, and at temperatures ranging from 500° C. to 700° C. Thecatalysts found to be the most effective for this process are thosewhich are based on potassium oxide (carbonate) promoted, chromium oxidestabilized, iron oxide material. Catalysts of this type are said to beself-regenerative inasmuch as, in addition to their effectiveness inpromoting dehydrogenation, they also promote the water gas reaction inthe presence of the steam co-feed, to thereby remove coke which wouldotherwise build up on and deactivate the catalyst. The lifetime of suchself-regenerative catalysts is thus determined by the effectiveness ofthe catalyst in maintaining its activity for conversion of ethybenzenematerials such as para-ethyltoluene for any given steam/hydrocarbonratio in the feed. Catalysts of this type which can maintain suchactivity at generally lower steam/hydrocarbon ratios are, of course,more economically desirable.

Considerable research has been directed toward attempts to improve theactivity and selectivity of iron oxide-potassium carbonate-chromiumoxide-based dehyrogenation catalysts. Any improvement which results ineither increasing the selectivity (moles of desired product per mole ofreactant reacted) or the conversion (moles of reactant reacted per moleof starting material) without lowering the other is economicallyattractive since the result is that the yield (moles of desired productproduced per mole of reactant) of the product has been increased. Anyincrease in the numerical value of the yield results in a more efficientoperation with more reactant being converted into the desired product.In commercial operations, many of which produce millions of pounds ofproduct per year, a trade-off is frequently effected between selectivityand conversion. An increase of only 1 or 2 percentage points in theselectivity can result in a substantial savings of starting materials.An increase in conversion can substantially reduce capital expenditureand energy consumption. The trade-off may vary depending on rawmaterials costs, energy costs, and the age of the plant.

Attempts have been made to improve the conversion effectiveness andselectivity of iron oxide type dehydrogenation catalysts for use invarious alkylaromatic dehydrogenation reactions. Riesser; U.S. Pat. No.4,152,300; issued May 1, 1979, for example, discloses that animprovement in ethylbenzene dehydrogenation catalyst selectivity can berealized by incorporating small amounts of certain metal oxide materialsinto dehydrogenation catalyst compositions comprising mixtures of ironoxide, potassium oxide, vanadium oxide and, optionally, chromium oxide.

Courty; U.S. Pat. No. 4,134,858; issued Jan. 19, 1979, discloses an ironoxide based dehydrogenation catalyst containing particular amounts ofclay to improve the conversion, selectivity and yield of styrene anddivinylbenzenes produced by dehydrogenation of ethyl- or diethylbenzene.This U.S. Pat. No. '858 patent also notes that oxides of copper,vanadium, zinc, manganese, magnesium, nickel, cobalt, bismuth, tin andantimony can be added to the disclosed dehydrogenation catalysts.

Notwithstanding such attempts to improve iron oxide baseddehydrogenation catalysts, there is a continuing need to formulatecatalysts of this type which can be used to realize improved conversionand/or yield in the dehydrogenation of other types of alkylaromaticmaterials such as, for example, in the production of para-methylstyrenefrom para-ethyltoluene.

Accordingly, it is an object of the present invention to provide animproved iron oxide based dehydrogenation catalyst especially useful forthe dehydrogenation of para-ethyltoluene to produce para-methylstyrene.

It is a further object of the present invention to provide apara-ethyltoluene dehydrogenation process employing a catalyst whichprovides desirably high conversion of para-ethyltoluene top-methylstyrene even at relatively low steam to hydrocarbon ratios inthe charge to the reaction zone.

These and other objectives can be achieved by means of the inventiondescribed and claimed herein.

SUMMARY OF THE INVENTION

The present invention relates to an improved dehydrogenation catalystcomposition especially useful for the selective dehydrogenation ofpara-ethyltoluene to produce para-methylstyrene. Such a catalystcomprises from about 30% to 60% by weight of an iron oxide component,calculated as ferric oxide, from about 13% to 48% by weight of apotassium compound component, calculated as potassium oxide, from about0% to about 5% by weight of a chromium compound component, calculated aschromic oxide, and from about 1% to 15% by weight of a magnesiumcompound calculated as magnesium oxide.

The present invention also relates to a dehydrogenation process whereinpara-ethyltoluene, along with steam, is passed over themagnesium-containing catalyst composition at a temperature from about500° C. to 700° C. with a LHSV of from about 0.3 to 3 and steam tohydrocarbon ratios of from about 1:1 to 5:1, to selectively producepara-methylstyrene.

DETAILED DESCRIPTION OF THE INVENTION

The dehyrogenation catalyst compositions of the present inventioncontain as an essential catalytic component one or more iron compounds,generally in the form of iron oxide. Many forms of iron oxide can beused in the catalyst compositions of this invention. Typically, ironoxides employed in catalyst preparations of this sort are asynthetically produced, powdered red, red-brown, yellow or blackpigment. The red or red-brown pigments are highly pure ferric oxide,while the black pigment is the magnetic form, ferrosoferric oxide (Fe₃O₄), which is usually found in the catalyst under various reactionconditions. The yellow iron oxides consist of the monohydrated form offerric oxide. These oxides are prepared by various methods, e.g.,oxidation of iron compounds, roasting, precipitation, calcination, etc.A suitable form of iron compound is the monohydrated yellow iron oxideused in the preparation of catalysts according to U.S. Pat. Nos.3,360,597, issued Dec. 26, 1967, and 3,364,277; issued Jan. 16, 1968.Particularly suitable are pigment grade red iron oxides of puritiesexceeding 98% weight. These red oxides have surface areas ranging from 2to 50 m.sup. 2 /gram and particle sizes from 0.1 to 2 microns. The ironcompound is present in the catalyst in either one or a mixture of bothof its possible oxidation states, i.e., as ferrous iron or ferric ironor mixtures thereof, as for example, ferrosoferric iron.

The catalyst compositions herein generally comprise from about 30% to60% by weight, preferably from about 35% to 55% by weight, of iron oxidecalculated as ferric oxide. Alternatively stated, the catalystcompositions herein generally comprise from about 21% to 42% by weight,and preferably from about 24% to 39% by weight, of iron oxide,calculated as iron metal.

The dehydrogenation catalyst compositions of the present invention alsoessentially comprise, as a catalyst promoter, one or more potassiumcompounds. The potassium promoter material can be added to the catalystin various forms. For example, it may be added as the oxide, or as othercompounds which are convertible, at least in part, under calcinationconditions, to the oxides, such as the hydroxides, the carbonates, thebicarbonates, the phosphates, the borates, the acetates, and the like. Aparticularly preferred potassium compound is potassium carbonate. Thepotassium compound is generally present in the catalyst as a potassiumoxide, a potassium carbonate or a mixture thereof. High carbon dioxidepartial pressures in the reaction gases will favor high carbonate tooxide ratios and vice versa within the potassium component.

The catalyst compositions herein generally comprise from about 13% to48% by weight, and preferably from about 27% to 41% by weight, ofpotassium promoter compound, calculated as potassium oxide.Alternatively stated, the catalyst compositions herein generally containfrom about 11% to 40% by weight, and preferably from about 22% to 34% byweight, of potassium oxide, calculated as potassium metal.

An optional, but frequently utilized, third component of the presentcatalyst composition is a chromium compound which serves as a stabilizerfor the active catalytic components. Chromium compounds have, in fact,typically been added to alkali-promoted iron oxide catalysts to extendtheir life. Chromium, as used in the compositions of this invention, canbe added to the catalyst in the form of a chromium oxide or in the formof chromium compounds which decompose upon calcination to chromiumoxides, as for example, chromium nitrates, hydroxides, acetates, and thelike. Chromium can also be added in the form of alkali metal chromates.If potassium chromates are used, such materials can, of course, alsocontribute to the requisite concentration of potassium essentiallypresent in the dehydrogenation catalyst compositions as hereinbeforediscussed.

Thus, the catalyst compositions herein can comprise from about 0% toabout 5% by weight, and preferably from about 1% to 4% by weightchromium compound, calculated as chromic oxide. Alternatively stated,the present composition can comprise from about 0% to 3.5% by weight,preferably from about 1.4% to 2.8% by weight, of a chromium oxidecalculated as chromium metal.

In accordance with the present invention, the dehydrogenation catalystcompositions containing iron, potassium and optional chromium compounds,as described, also essentially contain particular selected amounts of amagnesium compound which can provide magnesium oxide in the catalystcompositions herein after calcination. Addition of magnesium compoundsto the particular iron-potassium-chromium dehydrogenation catalystsutilized herein serves to enhance conversion of aromatic reactants whensuch catalyst compositions are used to promote the preferentialformation of para-methylstyrene from para-ethyltoluene.

Magnesium as used in the catalyst compositions of the present inventioncan be added to the catalyst in the form of magnesium oxide, MgO, or inthe form of other magnesium compounds which decompose upon calcinationto form magnesium oxide, as for example, magnesium carbonate, magnesiumhydroxide, magnesium sulfate, magnesium nitrate and magnesium acetate.Magnesium oxide itself, i.e., magnesite, is the preferred magnesiumcompound for use in formulating the compositions herein. Magnesiumcompounds are added to the catalyst compositions of the presentinvention to the extent of from about 1% to 15% by weight, morepreferably from about 2% to 10% by weight, calculated as MgO.

In addition to the foregoing materials, the catalyst compositions of thepresent invention can optionally contain a wide variety of materialssuitable for altering, adjusting or modifying the catalytic and/orphysical properties of such compositions. Materials, for example, whichcan act as stabilizers, activators, and promoters for dehydrogenationcatalysts of the type herein contemplated include, cobalt, cadmium,aluminum, nickel, cesium, and rare earths. Such additives can beincorporated in various forms including their elemental form or in theform of their oxides. If employed, such stabilizers, activators and/orpromoters generally comprise from about 1% to 15% by weight of thecatalyst compositions herein.

It should be noted that the compositions of the present invention neednot contain materials such as potassium aluminosilicate, e.g.,kaliophyllite, in order to enhance catalyst activity and/or selectivityto production of p-methylstyrene products. The magnesiumoxide-containing catalysts of the present invention, in fact, can bemaintained substantially free of clays or clay-like material withoutadversely affecting catalyst dehydrogenation activity or p-methylstyreneselectivity when used to promote dehydrogenation of p-ethyltoluene.

The physical strength of the catalyst compositions of the presentinvention can be improved, if desired, by adding any of a variety ofoptional insoluble binding agents. Binding agents can include, forexample, calcium aluminate and Portland cement. It should be noted thatbinding agents can contribute to the requisite magnesium content of thecatalyst compositions herein in the event that such binding agentscontain materials which form magnesium oxide upon calcination ofcomposites containing them. The density of the catalyst compositionsherein can likewise be modified by the addition of various fillersubstances, for example, combustible materials such as sawdust, carbon,wood flour, etc. Such materials can be added to the compositions duringpreparation and thereafter burned out after the catalyst pellets havebeen formed. Other porosity promoting aids include graphite and aqueoussolutions of methylcellulose, which also facilitate extrusion ofcatalyst pellets as hereinafter described. If employed, binders andother fillers generally can comprise up to about 20% by weight of thecatalyst composition.

The catalyst compositions of the present invention are in generalprepared by admixing the essential and desired optional components ashereinbefore described and by thereafter drying and optionally calciningthe resulting mixture. Calcination temperatures can thus range fromabout 100° C. to 600° C., preferably from about 150° C. to 550° C. Thecompounds of the catalyst compositions herein can be admixed in variousways. One method comprises ballmilling together a mixture of the desiredoxides and/or compounds decomposable upon calcination to oxides, addinga small amount of water, and extruding the paste formed to produce smallpellets, which are then dried and calcined. Another method is todissolve the components together, spray dry these components to form aresulting powder, calcine the powder into the resultant oxides, and thenadd sufficient water to form a paste which is extruded into pellets,dried and calcined. Another procedure involves precipitating thosematerials which are precipitatable, such as iron, chromium andmagnesium, as the resultant hydroxides, partially dewatering theresultant precipitate, adding soluble salts of the other desired metals,and then subsequently extruding, drying and calcining the resultingpellets. A preferred method involves dry-blend powdering of oxidesand/or compounds decomposable upon calcination to the oxides, addingwater, optionally containing dissolved therein soluble compoundsdecomposable upon calcination to the oxides, then mixing and/or mullingthe resultant paste, pelletizing the mixture, subsequently substantiallydrying at a temperature from about 50° C. to about 300° C., followed bycalcining the pellets to form the final product. The drying andcalcining could be carried out stepwise in the same furnace by suitableprogramming of the furnace temperature. Alternatively, water-insolubledry powders of oxides and/or compounds decomposable upon calcination tothe oxides are dry-mixed, and the balance of the other materials neededare dissolved in water and the resultant solution is used to form thepaste with the dry powders. There are many variations of the mixing ofdry powders, water and water soluble compounds that give equivalentresults and fall within the scope of this invention.

The catalysts of the present invention are especially effective inpromoting the dehydrogenation of para-ethyltoluene to selectivelyproduce para-methylstyrene. Such a dehydrogenation reaction is usuallycarried out at reaction temperatures of about 500° C. -700° C. However,higher or lower temperatures may be used without departing from thescope of this invention. The use of atmospheric, sub-atmospheric, orsuper-atmospheric pressure is suitable. However, it is preferable tooperate at as low a pressure as is feasible, and atmospheric orsub-atmospheric pressure is preferred. The process of the invention maybe carried out in batch, semi-continuous, or continuous operation, withcontinuous operation being preferred. The catalyst is employed in theform of a fixed bed, or in fluidized or suspended form. It is preferableto utilize a fixed bed. The reaction may be carried out in single stagereactors or by staging in series reactors. The reactors may be ofvarious designs, e.g., downflow reactors, radial reactors, etc.

With the use of the catalyst of this invention, it is essential to addsteam to the reactant feed to aid in the removal of carbonaceousresidues from the catalyst. The reaction feed generally contains from2-30 moles of steam for every mole of organic feed. Catalysts havinghigher potassium contents are usually employed at lower steam to feedratios. Steam to feed weight ratios of from about 1:1 to about 5:1 aredesirable. Good results are obtained with steam to feed ratios of about1.6:1 to about 4:1.

The contact time of the reactant-containing gas with the catalyst isusually defined in terms of liquid-hourly-space velocity (volume ofliquid hydrocarbon reactant per volume of catalyst per hour, i.e.,LHSV). The LHSV of the organic reactants according to this invention mayvary from about 0.3 to 3 and is preferably adjusted within this range toeffect the degree of conversion desired for the particular feed inquestion.

The catalysts of the present invention and their use will be furtherdescribed by the following illustrative examples which are provided forillustration and are not to be construed as limiting the invention. Itshould be noted that advantages resulting from increases of selectivityand/or conversion of only one or two percentage points are extremelysignificant in a commercial process which may produce many hundreds ofthousand pounds of product a day.

EXAMPLE I

A reference dehydrogenation catalyst is formulated by admixing thefollowing materials in the following concentrations:

    ______________________________________                                        COMPONENT    CONCENTRATION (WT %)                                             ______________________________________                                        Fe.sub.2 O.sub.3                                                                           46%                                                              K.sub.2 CO.sub.3                                                                           51%                                                              Cr.sub.2 O.sub.3                                                                            3%                                                                           100.0%                                                           ______________________________________                                    

To this mixture is added water to the extent of 20% and the resultingmixture is formed into a paste. The paste is formed into 3/8" diskswhich are then dried for 16 hours at approximately 180° C. The disks arethen crushed to 5-8 mesh particles.

EXAMPLE II

The dehydrogenation catalyst prepared as in Example I is used todehydrogenate para-ethyltoluene (PET) at atmospheric conditions to formpara-methylstyrene (PMS) in a tubular reactor maintained at 620° C. towhich PET and steam are introduced. Feed rates and steam/hydrocarbonratios are varied for several such dehydrogenation runs. Feed rates,steam/hydrocarbon ratios and the resulting conversion of p-ethyltolueneand selectivity to production of p-methylstyrene are set forth in TableI:

                  TABLE I                                                         ______________________________________                                        Dehydrogenation of Para-Ethyltoluene (PET) Over                               Reference Catalyst                                                            PET Feed         Conversion Results                                           Rate     H.sub.2 O/                                                                            PET Conversion                                                                             Para-Methylstyrene                              (LHSV)   PET     (Mole %)     Selectivity (Mole %)                            ______________________________________                                        1.0      2.0     57-59        88-90                                           1.0      1.85    54-56        88-90                                           0.4      1.85    55-57        86-89                                           ______________________________________                                    

The Table I data indicate that the unmodified reference catalyst isuseful for promoting dehydrogenation of p-ethyltoluene top-methylstyrene with relatively high selectivity to p-methylstyreneproduction.

EXAMPLE III

In a manner similar to that set forth in Example I, anotherdehydrogenation catalyst is prepared. The following dry materials areadmixed in the concentrations shown.

    ______________________________________                                        Component    Concentration (wt. %)                                            ______________________________________                                        Fe.sub.2 O.sub.3                                                                           44.2                                                             K.sub.2 CO.sub.3                                                                           49.0                                                             Cr.sub.2 O.sub.3                                                                            2.9                                                             MgO           3.9                                                                           100.0%                                                          ______________________________________                                    

Water is added to the extent of 20% and the resulting paste is againformed into 3/8" disks and dried for 16 hours at 180° C. The dried disksare then crushed into 5-8 mesh particles. The resulting MgO-containingcatalyst composition contains the same ratio of iron oxide to potassiumcarbonate to chromic oxide as does the reference catalyst of Example I.

EXAMPLE IV

Using the MgO-containing catalyst of Example III and the generalreaction conditions of Example II, para-ethyltoluene (PET) is againdehydrogenated to para-methylstyrene (PMS). The conversion results areset forth in Table II:

                  TABLE II                                                        ______________________________________                                        Dehydrogenation of Para-Ethyltoluene (PET) Over                               Magnesium Oxide-Containing Catalyst                                           PET Feed         Conversion Results                                           Rate     H.sub.2 O/                                                                            PET Conversion                                                                             Para-Methylstyrene                              (LHSV)   PET     (Mole %)     Selectivity (Mole %)                            ______________________________________                                        1.0      2.0     69-71        87-88                                           1.0      1.85    63-65        89-90                                           ______________________________________                                    

A comparison of the Table II data with that of Table I indicates thatthe modification of the reference catalyst with magnesium oxide inaccordance with the present invention gives a catalyst which providesimproved conversion of PET with no significant drop in selectivity top-methylstyrene production.

EXAMPLE V

Using the MgO-containing catalyst as described in Example III and thegeneral reaction procedures of Example II, p-ethyltoluene (PET) is againdehydrogenated to para-methylstyrene (PMS) at 620° C. over an extendedperiod of time with varying reaction conditions. Conversion results areset forth in Table III.

                                      TABLE III                                   __________________________________________________________________________    Dehydrogenation of Para-Ethyltoluene (PET) Over                               Magnesium Oxide-Containing Catalyst                                           Time on        H.sub.2 O/PET                                                                       Conversion Results                                       Stream (Hours)                                                                        PET LHSV                                                                             (Wt)  % PET Conversion                                                                        % PMS Selectivity                              __________________________________________________________________________    112      1.01  2.06  67.9      90.1                                           136      1.01  2.0   66.8      89.5                                           160     1.0    2.0   67.4      89.5                                           168     1.0    1.91  65.1      89.5                                           212     1.0    1.85  63.3      89.5                                           240     1.0    1.86  64.2      89.5                                           __________________________________________________________________________

The Table III data indicate that the MgO-containing catalyst of thepresent invention can effectively convert PET to PMS over an extendedperiod of time on stream at desirably low H₂ O/PET ratios.

What is claimed is:
 1. In a process of dehydrogenating para-ethyltolueneto selectively form para-methylstyrene comprising contacting thepara-ethyltoluene under dehydrogenation reaction conditions with acatalyst composition consisting essentially of:(a) from about 30% to 60%by weight of iron oxide, calculated as ferric oxide; (b) from about 13%to 48% by weight of a potassium compound, calculated as potassium oxide;and (c) from about 0% to 5% by weight of a chromium compound, calculatedas chromic oxide,the improvement wherein the process is conducted with acatalyst composition consisting essentially of, in addition to thecomponents (a), (b) and (c), a modifying component (d) capable ofenhancing the conversion of para-ethyltoluene when the modified catalystis used to provide the preferential formation of para-methylstyrene frompara-ethyltoluene, the modifying component (d) being a magnesiumcompound present to the extent of from about 1% to 15% by weight of thecatalyst composition, calculated as magnesium oxide.
 2. A process inaccordance with claim 1 wherein said dehydrogenation conditions includea temperature of from about 500° C. to 700° C., and a liquid hourlyspace velocity for para-ethyltoluene of from about 0.3 to
 3. 3. Aprocess in accordance with claim 2 wherein the catalyst compositionconsists essentially of:(a) from about 35% to 55% by weight of the ironoxide; (b) from about 27% to 41% by weight of the potassium compound;(c) from about 1% to 4% by weight of the chromium compound; and (d) fromabout 2% to 10% by weight of the magnesium compound.
 4. A process inaccordance with claim 3 wherein para-ethyltoluene is contacted with thecatalyst in the presence of steam and wherein the weight ratio of steamto para-ethyltoluene ranges from about 1:1 to 5:1.
 5. A process inaccordance with claim 4 wherein the catalyst composition issubstantially free of clay material.
 6. In a process of dehydrogenatingpara-ethyltoluene to selectively form para-methylstyrene comprisingcontacting the para-ethyltoluene under dehydrogenation reactionconditions with a catalyst composition consisting essentially of:(a)from about 30% to 60% by weight of iron oxide, calculated as ferricoxide; (b) from about 13% to 48% by weight of a potassium compound,calculated as potassium oxide; (c) from about 0% to about 5% by weightof a chromium compound, calculated as chromic oxide; and (d) up to about20% by weight of an insoluble binder/filler component selected from thegroup consisting of Portland cement, calcium aluminate, sawdust, carbon,wood flour, graphite, methylcellulose and mixtures thereof,theimprovement wherein the process is conducted with a catalyst compositionconsisting essentially of, in addition to the components (a), (b) and(c), a modifying component (d) capable of enhancing the conversion ofpara-ethyltoluene when the modified catalyst is used to promote thepreferential formation of para-methylstyrene from para-ethyltoluene, themodifying component (d) being a magnesium compound present to the extentof from about 1% to 15% by weight of the catalyst composition,calculated as magnesium oxide.